Natural gas is transmitted via high pressure pipelines and distributed to end users at considerably lower pressures. Generally, compressor stations are used to raise the pressure and to maintain it during long distance transmission. It is noteworthy that differing line pressures are used for transmission lines in differing geographical settings, and the pressures must be reduced accordingly in compliance with network design requirements in a varying number of steps, which depend upon the size and nature of the end user or subdistribution node on the system.
The process of pressure reduction is normally accomplished by means of a small orifice or throttling valve and results in a substantial lowering of the gas' temperature. Naturally, the extent of temperature drop is directly proportional to the extent of pressure reduction that occurs.
Temperature drop caused by Joule-Thompson processes is undesirable and must be avoided, or at least limited for a number of reasons. Excessive chilling can cause undesirable stresses in the pipes and ancillary equipment; it can degrade certain pipe coatings and pipe materials; it can also cause freezing of the earth surrounding the pipeline with the associated risk of frost heave. Furthermore, the gas itself may contain condensable components whose liquefaction or solidification in reduced temperatures may pose problems for the downstream network.
The most direct method for avoiding such problems is to heat the gas stream immediately before its pressure is reduced. The amount of heat delivered is controlled so that the post-expansion gas temperature remains high enough to circumvent low temperature problems upon pressure release.
Burning a portion of the gas represents a logical source of heat available to the natural gas pressure reduction station. Unless there is another reliable and uninterrupted source of heat available to the pressure reduction station, a bank of high efficiency gas fired boilers is usually deployed to provide the necessary heat. This remedy is effective and generally straightforward to implement, but it comes at the cost of consuming some of the deliverable energy in the gas. Proposals have been made to use fuel cells or combined heat and power (CHP) units rather than boilers to supply heat along with power, but the energy loss in terms of gas consumption still remains.
Prior art methods for reducing or eliminating the waste of energy in the process of pressure reduction in natural gas are described below.
U.S. Pat. No. 4,677,827 describes adding an inhibitor to the gas upstream of the pressure reduction. The purpose of the inhibitor is to prevent condensation in the chilled gas. After the inhibitor is added the pressure reduction is allowed to take place without preheating.
Reheating after pressure reduction can be accomplished by establishing thermal contact with the ambient since the expanded gas will generally have a temperature below ambient. This can be done in a number of ways. For example: by providing free refrigeration to an available load (provided that such a load can be found); by providing a direct or indirect heat exchange connection between the gas and the ambient or by supplementing passive heat exchange with heat supplied by a heat pump. These methods allow much if not all of the reheating to be supplied from the ambient, with a consequent saving in heat produced by gas burning.
Difficulties with this approach include the necessity to provide an additional consumable, i.e. the inhibitor, to each site and to meter its injection into the gas stream. In addition it may be necessary to recover the inhibitor before the gas is supplied to the end user. Recovery of the inhibitor entails additional equipment and adds materially to the complexity of the station and to its operation.
Pozivil (Acta Montanistica Slovaca, Rocnik 9 (2004), cislo 3, 258-260) reports transformation of the kinetic energy released in the gas expansion process into mechanical energy in an expansion turbine and, in most cases, subsequently into electrical power. This electrical power can then be used in a variety of ways: supplied back to the electricity grid; used to provide some or all of the electrical requirements of the site and possibly used to power a heat pump to supply heat to the expanded gas.
There are a number of issues to be addressed in considering the use of any of these power-generating methods. First is the fact that the gas temperature drop which accompanies a power-producing expansion is several times larger than that which accompanies a throttling expansion to the same final pressure. If this cooling is to be counteracted by burning gas upstream of the expander, the reheating process will consume more energy than can be generated even by the most efficient expander-generator unit. There must also be a full-time electrical load available to the station to utilise the electrical energy produced. In practical terms this usually means a grid connection through which the electricity is fed back into the network. In any case there is a net loss of usable energy even if the electricity generated is fully used. Justification for the expenditure for this arrangement must be sought from factors other than energy savings.
A variation of this approach is to use a CHP unit in addition to the expander-generator unit. The size of the CHP is determined by the amount of reheat required so that the thermal output of the CHP can be used to counteract the expansion-induced gas cooling. The electrical output of the expander-generator is added to that of the CHP unit and both are supplied to the grid. Both of the electrical outputs produce an economic return to the operator, but the primary energy and CO2 advantages of the approach are less straightforward to establish. The reason for deploying the CHP unit is mainly to take advantage of its thermal output, so this part of the combustion energy must be seen as sacrificial in the overall scheme. The role of the CHP could be replaced by a fuel cell, and the overall approach would be the same.
If the heat is to be added post-expansion, then it will be necessary to add condensation inhibitors to the gas stream. Indeed, because of the very large temperature drop it may be necessary to increase the dosage of inhibitor for it to remain effective. It will also be necessary to evaluate the implications for equipment of chilling by temperature drops down to −80° C. which can occur even in a single expansion stage. This method is capable of achieving significant primary energy savings, but its implementation presents in more extreme form all of the difficulties noted above in connection with the inhibitor addition method.
U.S. Pat. No. 5,628,191 communicates a system comprising a heat pump to heat the gas pre-expansion. Utilising the pre-expansion heat pump approach, one is faced with the problem of heating the gas up to temperatures as high as 80-90° C. from an inlet temperature typically of 5-10° C. so as to avoid the cooling problems discussed above (supra). Achieving the very high final temperatures is a Herculean challenge for any conventional heat pump to achieve. In addition, the necessity of achieving such a large temperature lift in a single pass will have a very deleterious effect on the heat pump efficiency. If the heat pump efficiency does not achieve a minimum threshold efficiency level, the process may still require supplementary (combustion) heating.
U.S. Patent Application Publication No. 2003/0172661 provides for use of multiple small-ratio expansion stages to restrict the temperature drops to a range which a heat pump could handle. Such an approach would entail much greater equipment cost and complexity, without any additional benefit. The above considerations taken together make it unlikely that conventional heat pumps can play any significant role in this particular application.
Notwithstanding the state of the art it would still be desirable to provide for a system that is capable of pre-heating a pressurised fluid to a sufficient extent such that upon fluid depressurisation the problems associated with cooling are avoided. It would be desirable that the system be energy efficient. Furthermore, a system capable of net power generation would also be desirable.